Hydrogenation of nitroarenes to anilines in a flow reactor using polystyrene supported rhodium in a catalyst-cartridge (Cart-Rh@PS)

Article abstract of DOI:10.1039/C8NJ04646H

The present methodology described the chemo-selective hydrogenation of various nitroarenes in a flow reactor under polystyrene supported rhodium in a catalyst-cartridge (Cart-Rh@PS) as a heterogeneous nano-catalyst. The polystyrene supported Rh (Rh@PS) nanoparticles (NPs) were prepared by following our earlier reported protocol and packed inside the catalyst-cartridge (Cat-Cart) to obtain Cart-Rh@PS, which is compatible with ThalesNano's H-Cube Pro flow system. The advantages of the prepacked catalyst Cart-Rh@PS are as follows: no need for catalyst activation up to 12 runs, negligible metal leaching detected by ICP-AES analysis and significantly less back pressure generated under the flow conditions. The same catalyst, Cart-Rh@PS, was also effective up to a 1 gram scale for the reduction of nitroarenes and reusable for successive runs. The hydrogenation in the flow reactor is a greener approach for the reduction of nitroarenes to their corresponding anilines in high yields.

Full text of DOI:10.1039/C8NJ04646H

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Hydrogenation of nitroarenes to anilines in a flow
reactor using polystyrene supported rhodium in a
catalyst-cartridge (Cart-Rh@PS)†
Cite this:DOI: 10.1039/c8nj04646h
ab
a
ab
Saurabh Sharma, Yamini and Pralay Das
*
The present methodology described the chemo-selective hydrogenation of various nitroarenes in a flow
reactor under polystyrene supported rhodium in a catalyst-cartridge (Cart-Rh@PS) as a heterogeneous
nano-catalyst. The polystyrene supported Rh (Rh@PS) nanoparticles (NPs) were prepared by following
our earlier reported protocol and packed inside the catalyst-cartridge (Cat-Cart) to obtain Cart-Rh@PS,
which is compatible with ThalesNano’s H-Cube Pro flow system. The advantages of the prepacked
catalyst Cart-Rh@PS are as follows: no need for catalyst activation up to 12 runs, negligible metal
leaching detected by ICP-AES analysis and significantly less back pressure generated under the flow
conditions. The same catalyst, Cart-Rh@PS, was also effective up to a 1 gram scale for the reduction of
nitroarenes and reusable for successive runs. The hydrogenation in the flow reactor is a greener
approach for the reduction of nitroarenes to their corresponding anilines in high yields.
Received 12th September 2018,
Accepted 10th December 2018
DOI: 10.1039/c8nj04646h
rsc.li/njc
Anilines synthesised through hydrogenation of nitroarenes are and heterogeneous transition metal catalysts such as Pd, Pt, Ni
important intermediates which are used for the production of and Co have been applied for the reduction of various functional
7
–10
dyes and polymers and are also useful in the pharmaceutical groups in a flow reactor (H-Cube) (Table 1).
These catalysts often
1
industry. Traditionally, aromatic amines have been synthe- suffer from limited substrate scope, backpressure generation, and
sized by the reduction of nitroarenes in the presence of various poor conversion and recyclability. However, heterogeneous rhodium
metal catalysts in combination with different hydrogenating catalysts were not screened for the reduction of nitroarenes under a
2
,3
4 3 2
sources such as NaBH , Et SiH, Ph MeSiH, etc. Under these flow reactor (H-Cube) and the cost of the commercially available
processes, huge amounts of inorganic or organic waste are Cat-Cart (THS01134-2EA (5% Rh/C)) is very high. There are few
generated; therefore, these methods are not atom-economical. reports that have been published on the reduction of nitroarenes by
The hydrogenation of nitroarenes using hydrogen gas in the utilization of heterogeneous rhodium catalysts including Rh/C
11a,b
presence of various transition metal catalysts can minimize the under conventional conditions.
inorganic/organic wastes which are produced from traditional In the last decade, we have been actively involved in the
methods and enhance the product yield. However, several development of new generation catalysts to reduce the cost, avoid
methods have been reported for the hydrogenation of nitro- the back pressure generated in the flow reactor, facilitate faster
arenes using hydrogen gas as a reducing source in the presence reaction, and improve catalytic efficiency and recyclability.
4
of different transition metal catalysts. But these methodologies
Herein, we explore our polystyrene supported rhodium in a
have some serious issues: they require a high pressure reaction Cat-Cart (Cart-Rh@PS) as a heterogeneous nano-catalyst for
system, an isolated place, and hydrogen cylinders and safety
precautions are needed to avoid leakage of gas. To overcome
these limitations, a flow reactor (ThalesNano’s H-Cube Pro) has
Table 1 Reduction of nitroarenes with various transition metal catalysts
2
been widely explored for in situ H production through electrolytic
decomposition of water and its concurrent applications in a
5
,6
reduction reaction. In the last few years, various homogeneous
Entry
Catalyst
Conditions (solv./temp.)
Ref.
a
Natural Product Chemistry and Process Development, CSIR-Institute of Himalayan
1
2
3
4
5
Maghemite–Pd
10% Pd/C
10% Pd/Al
ARP-Pt
RaNi
EtOH:EtOAc/30 1C–70 1C, H
EtOH:EtOAc/50 1C–90 1C, H
2
2
7a
7b
7c
8
Bioresource Technology, Palampur-176061, HP, India. E-mail: pdas@ihbt.res.in,
pralaydas1976@gmail.com; Fax: +91-1894-230-433
2
O
3
DMF/50 1C–70 1C, H
EtOH/25 1C, H
DMF/70 1C–100 1C, H
2
b
Academy of Scientific & Innovative Research (AcSIR), New Delhi, India
2
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj04646h
2
7c
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chemo-selective hydrogenation of nitroarenes in a flow reactor
2
(ThalesNano’s H-Cube Pro) using H as a reducing source. The
reaction mixture passed efficiently through Cart-Rh@PS, due to
the granulated form or shape of the Rh@PS catalyst and
negligible back pressure generated during the reaction course.
Most of the conversion completed in a single run, while in
some cases two or more cycles were required to achieve the
highest yield of the desired product. Further, the catalyst is
highly recyclable, is applicable for gram scale reaction, doesn’t need
work up and in a few cases doesn’t require product purification.
The Rh@PS catalyst was prepared by following our previously
reported reduction–deposition method (ESI†). We performed
the morphological studies of Rh on the Rh@PS catalyst before
packing in the Cat-Cart by scanning electron microscopy (SEM)
and transmission electron microscopy (TEM). The SEM image
Fig. 2 (a) SEM image of Rh@PS NPs at 10 mm. (b) SEM-EDX spectrum.
1
1
(Fig. 2(a)) and SEM-EDX spectrum (Fig. 2(b)) showed the dis-
persion and the presence of Rh metal on the polystyrene matrix.
TEM images in (Fig. 3(a and b)) at 20 and 5 nm, respectively,
confirmed that the particle size of the Rh NPs lies between 1.5
and 2.0 nm on the polystyrene matrix, and the distribution of
particles was demonstrated using a histogram measured for
419 Rh NPs using ImageJ software (Fig. 3(c)). The TEM image in
Fig. 3(d) analyzed by HRTEM equipped with a field emission
gun (FEG) described the crystalline feature of the Rh NPs, and
the d-spacing (0.224 nm) revealed that the crystal has a (111)
plane. A Fast Fourier Transformation (FFT) image of the selected
area (Fig. 3(e)) also showed a diffraction spot corresponding to
the fcc arrangement of the rhodium nanoparticles.
Fig. 3 (a) TEM image of Rh@PS at 20 nm. (b) TEM image at 5 nm.
c) Particle size distribution diagram of rhodium nanoparticles. (d) HRTEM
image at 1 nm showed the lattice fringe image spacing corresponding to the
0 mm size and the side was shielded using a mechanical packing (111) plane. (e) Fast Fourier Transformation (FFT) image of the selected area.
After the satisfactory synthesis of the catalyst, Rh@PS (350 mg,
mol% Rh) was filled into an empty catalyst cartridge (Cat-Cart)
(
2
7
system under vacuum using 8-micron Stainless Steel and Teflon
filters to obtain Cart-Rh@PS applicable with ThalesNano’s H-Cube
Pro system for the reduction of nitroarenes. In this process, H
2
a series of experiments by variations in the Rh@PS loading,
gas was generated through electrolytic decomposition of water flow rate and temperature. The reactant 4-nitrotoluene (100 mg)
in a ThalesNano’s H-Cube Pro flow reactor, mixed with the was taken in a mixture of solvents EtOAc/MeOH (1 : 1) and
reaction mixture and participated in a reduction reaction in passed through Cart-Rh@PS (Rh@PS: 350 mg, 2 mol% Rh) at
À1
Cart-Rh@PS (Fig. 1).
a temperature of 40 1C and a flow rate of 0.5 mL min at 60%
The optimization for hydrogenation was examined with
-nitrotoluene (1a) as a model substrate and investigated with a further decrease in the flow rate to 0.3 mL min with the
H
2
concentration; no product (2a) formation was observed. With
À1
4
2
same catalyst loading and H concentration at 40 and 60 1C, the
yield of product 2a was slightly increased (Table 2, entries 2 and 3).
Most remarkably at 80 1C, hydrogenation of 1a proceeded
efficiently and gave the desired product 2a in 50% yield
(Table 2, entry 4) in two runs. Increasing the temperature to
100 1C afforded the highest conversion (499%) of reactant 1a
and yielded product 2a (96%) in a single run. The flow rate (0.5–
À1
1
.0 mL min ) was found to be reversibly proportional to the
product yield, since under optimized conditions it reflected that
the interaction time with the substrate and catalyst is very
crucial for the fruitful conversion (Table 2, entries 6 and 7).
Subsequently, on lowering the percentage of Rh metal loading,
the yield of product 2a decreases (Table 2, entries 8–10). On the
other hand, in the presence of the Cart-Pd@PS catalyst, only
62% yield of product 2a (Table 2, entry 11) was observed. We
Fig. 1 Schematic representation of a flow reactor (H-Cube) for hydro-
genation reaction.
also packed 5% Rh/C in the Cat-Cart and applied the catalyst in
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Table 2 Optimization of the reaction conditions for 4-nitrotoluene (Table 3, entry 1). More importantly, full conversion of electron
a
reduction
donating substituted nitroarenes such as 2-nitrophenol (1c),
4-nitrophenol (1d), 2-methylnitrobenzene (1e), 2,6-dimethyl-
nitrobenzene (1f) and 4-methoxynitrobenzene (1g) revealed that
the electron donating substituents did not affect the reactivity
of the substrates and provided excellent yields of the corres-
ponding products 2c–2g (Table 3, entries 2–6). Moreover,
excellent chemo-selectivity was observed when substituents
containing reducible functional groups such as 4-cyanonitro-
benzene (1h) and 4-acetylnitrobenzene (1i) were subjected to
reaction under the present catalytic conditions, and they gave
excellent yields of products 2h and 2i in a single run at 40 and
60 1C respectively (Table 3, entries 7 and 8). Furthermore, the
methodology was also applied for the hydrogenation of halogen
substituted nitroarenes, viz. 2-chloro (1j), 3-chloro (1k), 4-chloro
(1l), 3,5-dichloro (1m) and 4-bromo (1n) nitrobenzenes, afforded
the corresponding amines (2j–2n) in 92–94% yield (Table 3,
entries 9–13), respectively, without the formation of dehalogenated
products. In our methodology, substituted ortho- and para-
nitrobenzenes were also targeted and the corresponding products
b
c
Temp. Flow rate
Conversion Yield
À1
Entry Catalyst (mol%)
(1C)
(mL min
)
(%)
(%)
1
2
3
4
5
6
7
8
9
1
1
Cart-Rh@PS (2)
Cart-Rh@PS (2)
Cart-Rh@PS (2)
Cart-Rh@PS (2)
Cart-Rh@PS (2)
Cart-Rh@PS (2)
Cart-Rh@PS (2)
Cart-Rh@PS (2)
Cart-Rh@PS (1)
40
40
60
0.5
0.3
0.3
0.3
0.3
0.5
0.7
1
0
3
0
2
8
5
80
55
499
61
15
10
20
3
50
96
57
8
7
18
2
100
100
100
100
100
0.3
0.3
0.3
0
1
Cart-Rh@PS (0.5) 100
Cart-Pd@PS (2) 100
70
62
a
All reactions were conducted with 4-nitrotoluene (1 equiv., 0.73 mmol),
Cart-Rh@PS (2 mol% i.e. 350 mg), EtOAc : MeOH (1: 1) as solvent, H
concentration 60% at 100 1C. Conversion based on GC-MS analysis.
2
b
c
Isolated yield.
2
o and 2p formed in excellent yields (Table 3, entries 14 and 15).
the same reduction reaction, but due to the high back pressure,
the reaction ended with incomplete conversion.
Using this methodology, 1,4-dinitrobenzene (1q) successfully was
hydrogenated into 1,4-diaminobenzene (2q) and gave 96% yield
at 100 1C (Table 3, entry 16) in a single run. The method was
found to be highly applicable for the reduction of nitroarenes
containing amide and acid groups as 3-nitrobenzamide (1r) and
To determine the best optimized reaction conditions, dif-
ferent H
2
gas concentration studies were performed under
À1
different flow rates (0.3, 0.5 and 0.7 mL min ) at 100 1C
(
0
Fig. 4). For 80–60% hydrogen concentrations at a flow rate of
.3 mL min , no significant decrease in the product yield was
4
-nitrobenzamide (1s) also were hydrogenated selectively into
the corresponding 3-aminobenzamide (2r) and 4-aminobenzamide
2s) without hydrogenation of the amide group at 90 1C in two runs
Table 3, entries 17 and 18), whereas 2-nitrobenzoic acid (1t) was
also reduced smoothly into the desired amine 2t in a single run
Table 3, entry 19). Gratifyingly, 1-(benzyloxy)-4-nitrobenzene (1u)
À1
noticed. But on further decrease of the hydrogen concentration
(
(
À1
from 40 to 20% at a flow rate of 0.3 mL min , a 3–7% decrease
in the yield of product (2a) was noticed (Fig. 4).
By screening the above results, we found that the best
(
optimized conditions are: Cart-Rh@PS (Rh@PS: 350 mg,
and 1-nitronaphthalene (1v) were also reduced under the standard
reaction conditions and delivered 4-(benzyloxy)-aniline (2u) and
naphthalen-1-amine (2v) successfully in a single run (Table 3,
entries 19 and 22). Electronically rich heterocyclic nitroarenes were
hydrogenated successfully into the corresponding amines without
hydrogenation of C–C and C–N double bonds and gave excellent
yields of products 2w–2z (Table 3, entries 22–25). The reduction of
nitroarenes followed the reported pathway like that of nitro-
benzene to nitrosobenzene then hydroxylamine and finally aniline
À1
2
2
mol% Rh), 60% H concentration, flow rate 0.3 mL min
at a temperature of 100 1C for reduction of 4-nitrotoluene (1a)
to product 2a in 96% yield (Table 2, entry 5).
We explored the standard reaction conditions to investigate
the diversity of the present protocol for the hydrogenation of various
substituted nitroarenes and the results are summarised in Table 3.
Under this protocol, nitrobenzene (1b) was hydrogenated at
100 1C in a single run and gave an excellent yield up to 98%
(
2a) in the presence of the Cart-Rh@PS catalyst under hydrogen
11a
gas as the reducing source.
Further, we explored the developed protocol for 1 gram level
synthesis, and the same quantity of catalyst (Cart-Rh@PS) was
found to be effective for the reduction reaction. In this study,
different nitroarenes were selected on a 1 gram scale and gave
the corresponding reduced products 2a, 2d and 2h in 93, 94
and 85% yields, respectively, successfully (Scheme 1).
The recyclability of the catalyst (Cart-Rh@PS) was screened
by the reduction of 4-nitrotoluene (1a); after completion of
the reaction, the cartridge was flushed with methanol prior to
further use and then repeatedly used for the same reaction. We
found that the catalyst can be recycled in up to 12 runs without
any significant loss in catalytic activity for the reduction reaction
(Fig. 5).
Fig. 4 Percentage of H
conversion based on GC-MS analysis.
2
concentration vs. percentage of conversion,
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a
Table 3 Synthesis of substituted anilines
Table 3 (continued)
b
b
Entry Substrate
1
Temp. (1C) Product
Yield (%) Entry Substrate
Temp. (1C) Product
Yield (%)
100
98
18
90
93
2
3
4
80
80
80
96
95
96
1
2
9
0
80
90
92
85
2
1
80
96
5
80
98
2
2
2
2
3
4
80
80
90
95
94
95
6
7
8
9
40
40
60
80
97
97
94
93
2
5
80
90
a
Reaction conditions: substituted nitroarenes, 1a–1z (1 equiv.), and
b
Rh@PS (2 mol% Rh), EtOAc/MeOH. Isolated yields.
1
1
1
1
1
0
1
2
3
4
100
100
60
95
94
93
92
97
a
Scheme 1 Gram scale synthesis of substituted anilines, isolated yields.
80
80
1
2
5
6
100
100
93
96
Fig. 5 Recyclability of Cart-Rh@PS for reduction of 4-nitrobenzene.
1
7
90
90
We also studied the efficiency of the catalyst and found that
the turnover number of the catalyst is 728 and the turnover
À1
frequency is 3.03 min . The Cart-Rh@PS catalyst was also
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Conclusions
In this methodology, we efficiently reduced various nitroarenes
to the corresponding aromatic amines by using molecular H
2
gas in the presence of Cart-Rh@PS as a catalyst under flow
reaction conditions (ThalesNano’s H-Cube Pro). The developed
protocol was applicable to a vast substrate scope and the catalyst
is recyclable for several successive runs. The Cart-Rh@PS catalyst
was found to be efficient for activation of molecular hydrogen gas
and simultaneously participated in the reduction reaction. The
added advantages of the process are as follows: a shorter reaction
time is required, there is no need for tedious product purification
in a few cases, and there is no need for catalyst separation and
activation. The same catalyst, Cart-Rh@PS, was also applicable
for gram scale reduction reaction successfully, opening up scopes
for industrial application.
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2
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